Polymerizable liquid crystal materials are known in prior art for the preparation of anisotropic polymer films with uniform orientation. These films are usually prepared by coating a thin layer of a polymerizable liquid crystal mixture onto a substrate, aligning the mixture into uniform orientation and polymerizing the mixture.
For specific applications it is required to induce planar alignment in the liquid crystal layer, i.e. where the liquid crystal molecules are oriented substantially parallel to the layer. The alignment is then frozen in by polymerizing the liquid crystal mixture in situ. For example, oriented films or layers of polymerized nematic liquid crystal material with planar alignment are useful as A-plate compensators or polarizers. Another important application are oriented films or layers of polymerized cholesteric liquid crystal material having twisted molecular structure. If the cholesteric material has planar alignment, these films show selective reflection of light where the reflection color is dependent on the viewing angle. They can be used for example as circular polarizers, color filters or for the preparation of effect pigments for decorative or security applications.
Planar alignment can be achieved for example by treatment of the substrate onto which the liquid crystal material is coated, such as rubbing or application of alignment layers, or by applying shear forces to the liquid crystal material, for example during or after coating.
It is also known in the prior art that planar alignment of a liquid crystal material on a single substrate, with one surface of the liquid crystal material being open to the air, can be achieved or enhanced by addition of a surface active compound to the liquid crystal material.
WO 99/45082, for example, describes an optical retardation film that is obtained from a layer of polymerizable liquid crystal material with planar alignment comprising one or more fluorocarbon surfactants. U.S. Pat. No. 5,995,184 reports a method of making a phase retardation plate from a layer of polymerizable liquid crystal material with planar alignment, where a surface active material, for example, a polyacrylate, polysilicone or organosilane, is added to the liquid crystal material to reduce the tilt angle at the liquid crystal/air interface of the liquid crystal layer.
However, the methods to achieve planar alignment described in the above documents have several drawbacks. The surfactants often tend to phase separate from the liquid crystal material during or after polymerization. Also, if another layer is provided onto the polymerized liquid crystal film comprising the surfactant, the surfactant tends to migrate into the second layer. For example, in specific applications, such as cholesteric color filters, a film of polymerized cholesteric liquid crystal material is part of a composite LC cell, in which other layers need to be added to the cholesteric film, such as, but not restricted to, other liquid crystal, retardation or topcoat layers. During the addition of the subsequent layers conventional surfactant molecules are leached out of the bottom layer and migrate to the air interface. This results in a sample/air interface region which becomes progressively more concentrated in surfactant. Hence, subsequent layers have different surface energies and can no longer be thought of as being identical. The migration of surfactant can also cause a problem where it is desired to place a top layer which does not contain a surfactant, or where the surfactant can also cause other species to migrate from one layer to the next.
It is therefore an aim of the present invention to provide a polymerizable liquid crystal material for the preparation of a polymer film with planar alignment that does not have the drawbacks described above. Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.